Kyoto Protocol officially came into effect in February 2005 to take measures against global warming, and planning and execution of effective measures for reduction in CO2 have attracted worldwide attention. Under such circumstances, it is greatly expected to practically use biomass energy as recycled and new “carbon-neutral” energy. The “carbon-neutral” energy means that since the wood, agricultural crop and plant serving as fuels absorb CO2 in the atmosphere through photosynthesis in their growing processes, even when CO2 is produced in a burning process to create energy, CO2 is not increased in the atmosphere substantially.
Methods of producing the biomass energy include a method of generating electricity (thermal and electric energy) by burning, a method of producing gas (mixed gas of methane, hydrogen and carbon monoxide) by partial burning, and a method of generating methane gas or ethanol fuel by fermentation. When the wood or agricultural crop such as sugarcane, corn, and chaff (or its residue) is used as a fuel in each of the above production methods, unnecessary and spent fuel (biomass cascade) is always left after the energy has been extracted.
Focusing on the biomass energy that is practically used at the present, most part of various kinds of biomass cascade such as residue provided after ethanol has been refined from an agricultural crop in America and Brazil, combustion ash provided after chaff has been burned for electric generation in Thailand is disposed by a for-profit waste disposal service and only a part of it is given back to a farmland as fertilizer or used as a cement admixture. However, since these prices are very low such as several tens of yen per ton in Japanese yen, the cost for transporting the cascade to the farmland or a cement factory is equal to the cost for the above for-profit waste disposal service, which is a serious negative factor in the business using the biomass energy.
For example, Japanese company, TSUKISHIMA KIKAI CO., LTD. developed a technique to produce ethanol fuel from hexose such as cellulose or starch and pentose such as hemicellulose contained in a fuel such as a wood or corn by hydrolyzing the fuel with diluted sulfuric acid and decomposing glucose with enzyme and fermenting ethanol with yeast (adapted from “Chemical Equipment” issued in July 2003, p. 69-73). In addition, Sojitz Research Institute developed a technique for producing fuel ethanol by fermenting ground rice produced in Vietnam (adapted from Chemical Industry Report on Jul. 22, 2005). In either biomass technique, biomass cascade as residue after sugar has been extracted is compressed and solidified and recycled only as a solid burning fuel for a plant.
Meanwhile, a method for extracting high-purity silicon oxide from silicon oxide in a biomass fuel has been studied and developed. For example, a method for producing silicon oxide by burning chaff that is one of the agricultural crops and thermally decomposing cellulose, hemicellulose, and lignin has been proposed. For example, the methods are disclosed in Autumn Proceedings of Journal of the Japan Society of Powder and Powder Metallurgy in 2004: Process Design on Active Reuse of Chaff Ash by Solid-Phase Synthesis Method (p. 148, Katsuyoshi Kondoh, et al.), Spring Proceedings of Journal of the Japan Institute of Metals in 2005 (Mar. 28, 2005): Solid-phase Recycle Technique on Magnesium Alloy (Katsuyoshi Kondoh), and ICMAT 2005 & IUMRS-ICAM2005 (Jul. 6, 2005): Innovative Reuse of Agricultural Wastes as Industrial Raw Materials to Form Magnesium Composites (Katsuyoshi Kondoh et al.).
According to the above burning methods, since alkaline metal element contained in the chaff, especially potassium (K) reacts with silicon oxide in a heating process and carbon (C) contained in cellulose and/or hemicellulose is introduced in silicon oxide as residual carbon, purity of silicon oxide is as low as 88 to 90%. Meanwhile, when the chaff is burned at 800 to 900° C. or more, since cellulose and/or hemicellulose is completely decomposed and removed, a silicon oxide content can be 95% or more. However, such heating at a high temperature within the above range causes silicon oxide to be crystallized. The crystallized silicon oxide is not only inferior in function as compared with amorphous silicon oxide but also adversely affects a human body. According to IARC (International Agency for Research on Cancer) that is an affiliated organization of WHO (World Health Organization), since crystalline silica belongs to carcinogens (group 1), it is restricted in use similar to asbestos. Therefore, it is difficult to produce high purity amorphous silicon oxide from the biomass by the above burning method.
In addition, a method of producing high-purity silica from chaff has been proposed. For example, Japanese Examined Patent Publication No. 7-57684 discloses “production method of high-purity silica from chaff”. According to the method disclosed in this document, chaff is treated with a solution of hydrofluoric acid (HF) to extract a silicon compound contained in the chaff to produce silica. More specifically, the silicon compound provided after the treatment with the solution of hydrofluoric acid is extracted as hexafluorosilicic acid (H2SiF6) and the extract is added with ammonia and decomposed via ammonium hexafluorosilicate ((NH4)2SiF6) into ammonium fluoride (NH4F) and silica (SiO2), and this is separated by filtration, washed in water and dried to obtain amorphous silicon oxide powder. According to this method, the problem is that economic efficiency is lowered because expensive hydrofluoric acid is used and an equipment to be used needs to be made of an expensive material because hydrofluoric acid has strong corrosive aggressiveness.
Meanwhile, a production method of a material with a high silica content from a wood such as chaff has been proposed. For example, Japanese Unexamined Patent Publication No. 11-323752 discloses “production method of material with high contents of silica, cellulose and lignin”. According to the method disclosed in this document, after blasting or steaming treatment of a wood such as chaff, rice straw or bagasse (the wood is immersed into a solution such as hydrochloric acid, sulfuric acid or nitric acid and heated and pressurized), the wood is burned at 550 to 900° C. (more preferably, at 600 to 700° C.), whereby amorphous silicon oxide having SiO2 content of 95% is produced. In other words, it is necessary to thermally dissolve and remove an impurity by heating and burning at a higher temperature to produce high-purity silicon oxide having purity of 96% or more. However, the problem is that such high-temperature heating causes a crystal structure of the silicon oxide to be crystallized (become cristobalite).
Furthermore, a production method of amorphous silica fine powder from chaff has been proposed. For example, Japanese Unexamined Patent Publication No. 5-194007 discloses “production method of amorphous silica fine powder and concrete product blended with amorphous silica fine powder”. According to the method disclosed in this document, amorphous silica fine powder is produced by immersing chaff in a solution of hydrochloric acid and burning and grinding it. More specifically, the chaff is immersed in a hydrochloric acid solution having a concentration of about 10% for a short time to melt an alkaline organic element such as potassium, and burned at 600 to 700° C. Here, when it is burned at a temperature above 800° C., the problem is that silicon oxide becomes cristobalite (crystallized). Meanwhile, when it is burned at a temperature below 600° C., unburned carbon remains and purity of the silica powder is lowered. In other words, since the burning temperature of the chaff after treated with acid is limited to the above 600 to 700° C. and the temperature management range is small, when it is produced actually, quality varies-due to unburned carbon and crystallization of silica.
Furthermore, a production method of high-purity amorphous silica from chaff has been proposed. For example, Japanese National Publication of PCT Application No. 2004-527445 discloses “production method of high-purity amorphous silica from biogenic material”. According to the method disclosed in this document, a long-chain hydrocarbon compound such as lignin or cellulose is reduced to further combustible (volatile) short-chain hydrocarbon compound by immersing a biogenic material such as chaff in a solution having an oxidizing solute (selected from a group comprising peroxide, hydrogen peroxide, nitrate salt and permanganate) to reduce an amount of carbonaceous residual material of silica produced by the next process of burning. The burning temperature at that time is preferably 500 to 950° C., and crystallization of silica is accelerated in the high-temperature range.
In order to spread the biomass technique widely, it is necessary to reduce the total cost of the biomass energy by producing a high-value added product from the biomass cascade left as residue. More specifically, focusing on silicon oxide mainly contained in the cascade, a production method of amorphous silicon oxide having purity of 96% or more is to be established.